1. Field of the Invention
Aspects of the present invention relate to a fuel cell, and more particularly, to a fuel cell including a stack with a built-in heat exchanger having a structure to advantageously reduce space occupancy of the fuel cell.
2. Description of the Related Art
In general, a fuel cell is an apparatus that directly transforms chemical energy of a fuel into electrical energy through a chemical reaction. A fuel cell can be described as a type of electric generator that can generate electricity as long as fuel is supplied.
FIG. 1 is a schematic drawing showing the principle of generating electricity from a typical fuel cell, FIG. 2 is a perspective view of a conventional fuel cell stack having a heat exchanger located external to the conventional fuel cell stack, and FIG. 3 is an exploded perspective view showing a configuration of material circulation parts of a unit cell 10 included in a stack of fuel cells. Referring to FIG. 1, electricity is generated by a reverse reaction of electrolysis of water through an electrolyte membrane 2 when air containing oxygen is supplied to a cathode electrode 1 and hydrogen is supplied to an anode electrode 3. However, the electrical voltage generated from a unit cell 10 is generally not high enough to be useful. Therefore, as depicted in FIG. 2, a plurality of unit cells 10 are arranged in a stack 20 in which the plurality of unit cells 10 are connected in series. As depicted in FIG. 3, surface flow channels 4a are included in each cell 10 stacked in the stack 20 to supply hydrogen and oxygen to each respective anode electrode 3 and cathode electrode 1 and recover the oxygen and hydrogen. Accordingly, as depicted in FIG. 2, when hydrogen or oxygen is supplied through an end plate 21 of the stack 20, corresponding fuel materials are circulated to each of the electrodes through respective flow channels of each cell 10 and exhaust material are carried away. As described above, hydrogen is supplied as a chemical fuel, and oxygen is supplied from the air. Exhaust materials may be water, carbon dioxide, and unreacted fuel.
During the electrochemical reaction, heat is generated as well as electricity. Therefore, smooth operation of a fuel cell requires heat to be continuously removed from the cells. For this purpose, a heat exchanger 30 as shown in FIG. 2 is provided externally, in conjunction with the fuel cell, and cooling plates 5 for passing cooling water for exchanging heat energy are formed in every fifth or sixth unit cell 10 in the stack 20. Accordingly, the cooling water absorbs heat from the stack 20 while passing through flow channels 5a (see FIG. 3) of the cooling plates 5. The cooling water that absorbed heat is cooled in the heat exchanger 30 by secondary cooling water, and is then re-circulated to the stack 20. At this time, the circulation of the cooling water is achieved by natural convection of boiling water that has absorbed heat from surroundings in the fuel cell stack 20 and not by an additional circulation force. Reference numeral 40 denotes a thermo-sensor that measures the temperature of cooling water entering into the heat exchanger 30, and reference numeral 50 denotes a solenoid valve that opens and closes a flow channel from the heat exchanger 30 to the stack 20. During normal operation, the solenoid valve 50 is opened to allow the circulation of cooling water, but when the temperature of the cooling water entering into the heat exchanger 30 is too low, the solenoid valve 50 closes the flow channel so that the temperature of the cooling water in the stack 20 can increase. Afterward, the solenoid valve 50 is opened when the temperature of the cooling water in the stack 20 has increased to a desired level. The opening and closing of the solenoid valve 50 is automatically controlled by a controller (not shown).
However, in a conventional fuel cell structure, the stack 20 and the heat exchanger 30 are mounted separately. Therefore, the overall volume of the fuel cell is large, and thus occupies a large space. That is, space required for mounting a fuel cell in an apparatus that uses the fuel cell is increased since the stack 20 and the heat exchanger 30 which respectively occupy separate spaces are both required. This is a drawback of the application of conventional fuel cells. Also, flow channel lengths for the cooling water are increased according to an increase in the volume of the conventional fuel cell, thereby increasing heat loss. This results in loss of the waste heat generated during the electricity production of the fuel cell for secondary purposes as well. Therefore, there is a need to develop a fuel cell having a new cooling system that has a compact structure to overcome the above and/or other disadvantages.